Tag Archives: flow

The thick and thin of viscosity

Viscosity is a liquid’s property of resisting deformation, such as flow. At home, you probably know this property as the ‘thickness’ of a fluid. The word comes from the Latin word “viscum”, meaning ‘mistletoe’, the berries of which were used in ancient times to create glue.

Water doesn’t behave like that — that’s viscosity.
Image via Pixabay.

Viscosity can be a weird concept to wrap your head around, because it makes fluids act not like fluids. Such a large part of our intuitive understanding of fluids relies on them behaving a certain way. Water, the quintessential liquid, flows immediately when poured; it takes the shape of its container, and if there’s no container, it spills everywhere. It gets absorbed by sponges or soil; it bubbles with gusto when boiling.

On the other hand, pitch can easily be confused for a solid. Because it’s so highly viscous, it barely flows, has quite an easy time maintaining its rough shape, and barely percolates through either soil or sponges.

So let’s see what this property which makes fluids un-fluid-like is.

The source of viscosity

In strictly physical terms, viscosity is a fluid’s shear stress over its shear rate, and is expressed in poise (P), a measure of pressure per second. It shows how much force per unit of area you need to apply to make a fluid move (shear stress) and how the internal layers of that fluid will move in respect to one another (shear rate).

In short, it shows how much energy you need to apply to make a fluid flow. The field of science that studies the flow patterns of matter is called rheology from the Greek term “rheo”, meaning ‘flow’.

Viscosity is the product of internal friction between the fluid’s molecules. Both gases and liquids have viscosity, but the molecules in liquids are packed in more tightly — making them interact more and, thus, have higher viscosity than gases.

Viscosity makes fluids flow more slowly, but also stick strongly to solids.
Image via Pixabay.

Shear rate is important in this equation because it describes how successive ‘layers’ of molecules in fluid interact with one another. A great way to see it in action is to try pouring honey out of a jar. It flows much more slowly than water because it is more viscous than it. But you’ll also notice that the honey in the center will flow out before the honey on the sides of the jar does as well.

This is due to the friction between the layers we mentioned earlier. Honey molecules that are right next to the glass will be attracted to the molecules in glass strongly enough that they’re basically stationary (solids are dense bunches of immobile molecules). They will generate friction with other honey molecules, in turn.

Friction always opposes the direction of motion so, in effect, this stops the fluid from sliding off the glass. Further layers of honey also experience friction, but against layers of fluid that are more mobile than the glass — so there’s less friction. Keep the model going and honey right in the middle of the jar will be most flowy because it’s experiencing little friction (against other relatively mobile molecules of honey).

It doesn’t only slow down pouring fluid — viscosity also prevents objects from passing through. A spoon will go through water with almost no effort, but not so through honey.

File:Air bubbles. Honey.jpg
Air bubbles rise quickly through soda, but have a hard time ascending though honey.
Image via Wikimedia user Sichnoy.

A good way to think about viscosity is that all the molecules in a fluid have hands, and they’re using them to hold onto their neighbors. The better these molecules can latch on, the more viscous the substance will be. In physical terms, these hands make up the cohesive force, being generated by chemical and physical interactions between molecules.

Types of viscosity

True solids cannot be vicious. In solids, molecules cannot really move in relation to one another. If one moves, they all move, or break apart. Since viscosity is defined in relation to flow (where particles move independently from one another), it can’t by definition be applied to a true solid. It would be like trying to define what light tastes like. That being said, not all fluids are born equal in regard to viscosity.

The most common class of viscous material you’ll encounter are Newtonian fluids. These always behave like fluids should, having a relatively constant viscosity. Although their resistance to flow does increase as more force is applied, this is proportional to the force being applied. Newtonian fluids, for all intents and purposes, keep acting like fluids no matter how much force you exert on them.

Air is a Newtonian fluid, although viscosity has no bearing on the speed of sound through air.

Hitting a balloon filled with colored water (bottom) makes it splatter everywhere. A mixture of starch and water, however (top, a non-Newtonian fluid,) becomes almost solid-like and breaks apart. Also, note how much each slows the bat on impact.
Image credits Reddit user u/Lord_Rae.

On the other end of the spectrum, we have non-Newtonian fluids. With them, viscosity varies very strongly with the force being applied, but not necessarily in the same way between different materials. Silly putty is a good example; left to itself, it’s somewhat flabby, but when you apply force against the putty, it hardens up. There are three large categories of non-Newtonian fluids: dilatant (apparent viscosity increases with exerted force), pseudoplastic (apparent viscosity decreases with exerted force), and generalized Newtonian fluids (which, ironically enough, are non-Newtonian). Ketchup is also non-Newtonian, but it becomes less viscous when force is applied.

Electrorheological and magnetorheological fluids increase their viscosity in the presence of electrical and magnetic fields respectively and can become near-solid under the right conditions, and quickly reverting back to a liquid if needed.

What has viscosity ever done for you?

4b46au
A mixture of corn starch and water (similar to the previous one) exhibiting non-Newtonian properties.
Credits Youtube / ScienceMandotcom.

Perhaps one of the most important impacts viscosity has on our lives is in our biology. The size, shape, and structure of our hearts, circulatory system and virtually every other tissue, as well as the structures and functionality of our cells, are all shaped to take advantage of or work around viscosity. Our immune system relies in no small part on our lymph channels. Lacking the propelling power of our heart, lymph vessels have to be designed around the viscosity of lymph to keep everything in working order.

Each morning, when you go brush your teeth, it’s viscosity preventing the paste in the tube from splattering everywhere. Without viscosity, your ketchup would drain out of the bottle almost instantly, and the lubricants in your car wouldn’t be able to coat (and thus, lubricate) anything.

Viscosity is what makes liquids want to pool together, and it was a key player in applications such as pill manufacturing. Industries involved in the production of food, chemicals, adhesives, biofuels, paints, medicine, and petroleum processing all keep a close eye on the viscosity of their products.

Finally, viscosity is always affecting us, although we’ve evolved to not feel it. But whenever you make even the slightest of motions, the air’s viscosity works against it — it produces ‘air drag’. Swimming, too, lets you experience a somewhat more concentrated flavor of viscosity.

Personally, I find that processes happening mostly in the background of our awareness, the unsung underdogs of physics (such as viscosity) are the most fascinating ones to study. We may take them for granted, we may not even be aware they exist, but they silently play a part in everything we do. Even a ‘humbler’ one, like viscosity, has directly shaped life on Earth into what it is today.

NASA puts InSight experiment on hold because one stubborn rock is blocking their instruments

A key instrument on NASA’s Mars InSight rover has run into a problem — ground control suspects a stone.

Mars landing.

A rendering of a InSight Mission Candidate Landing Site made using topography data from the University of Arizona / NASA/.
Image credits Kevin Gill / Flickr.

The rover’s heat probe has struck an obstacle just below the red planet’s surface over the weekend and hasn’t been able to make progress since.

The Heat Flow and Physical Properties Package Problem

“The team has therefore decided to pause the hammering for about two weeks to allow the situation to be analyzed more closely and jointly come up with strategies for overcoming the obstacle,” Tilman Spohn, the principal investigator for the heat probe, wrote Tuesday in the mission logbook.

The instrument, known as the Heat Flow and Physical Properties Package, or HP³, was designed to hammer itself 16 feet (roughly 5 meters) into Mars’ underground and measure how much heat its interior leaks. This data would help researchers estimate the planet’s composition and history.

However, trouble is brewing underneath InSight — this probe (nicknamed the “mole”) encountered some kind of resistance underground over the weekend and hasn’t been able to make any progress since. Ground control (at the Jet Propulsion Laboratory in La Canada Flintridge, California) first tried to power it up last week. This first attempt failed to reach all the way to the Mars Odyssey orbiter, however, which was supposed to pass it on to InSight.

The mole was deployed last Thursday, after the team established a stable connection to the rover. It pushed its way in the red soil and made quick progress. For about five minutes. The next four hours of hammering failed to push the mole much deeper and eventually forced the device to one side — the mole is now lodged in the underground, leaning at about 15 degrees of vertical.

InSight.

Artist’s concept of InSight and its instruments.
Image credits NASA / JPL-Caltech.

Current estimates place the mole at a depth of around one foot (0.3 meters). This means that the probe — measuring some 16 inches (0.4 meters) in height — is partially sticking out of the ground. Despite this, the probe likely still is burrowed “deeper than any other scoop, drill or probe on Mars before,” which was its intended purpose.

Spohn writes that the team is a bit worried but that they “tend to be optimistic.” They’re currently working on the assumption that the holdup is a buried boulder or some gravel.

This particular spot was picked for InSight to land on as it appeared to be mostly sandy and soft. However, the team was aware that such a holdup was possible. Tests carried out at JPL suggested that the probe should be able to dig its way around small rocks or layers of pebbles. Since the second attempt to hammer away at the probe didn’t do that, the team decided to put the mole on hold. They’re currently waiting to receive more data from InSight, including pictures, so they can “better assess the situation.”

But not all is lost. The probe is still intact — that’s a really good thing — so it can actually start collecting data. The team has already put it to the task. HP³ will measure how quickly a generated pulse of heat spreads through the soil. Later this week, as (Mars’ moon) Phobos passes overhead and eclipse the sun over InSight the probe will also track how the event changes surface temperatures. While not its intended role, these readings should help the team make better sense of heat flow values in Mars’ soil if and when the probe is deployed as planned.

Hand Plasma Lamp.

New theoretical framework will keep our fusion reactors from going ‘boom’

New theoretical work finally paves the way to viable fusion reactors and abundant energy for all.

Hand Plasma Lamp.

Hand touching a plasma lamp.
Image credits Jim Foley.

A team of physicists from the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) at Princeton University’s Forrestal Campus, New Jersey, may have finally solved a long-standing problem in physics — how to tame fusion for energy production. Their work lays down the groundwork needed to stabilize the temperature and density levels of plasma in fusion reactors, an issue that plagued past efforts in this field.

Wild and energetic

Plasma is one of the four natural states of matter. That may sound confusing since we’re all thought that stuff is either a gas, a liquid, or a solid, but there’s a good explanation for this: plasma is such a violent and energetic state of matter that it simply doesn’t exist freely on Earth. It is, however, the stuff that most stars are made of.

Think of plasma as a soup, only instead of veggies, it’s full of protons and electrons (essentially, highly-energized hydrogen atoms) that smack together to create helium. For a less-culinary explanation, see here. This process requires a lot of energy to get going — you need to heat the hydrogen to about 100 million degrees Celsius — but will generate monumental amounts of energy if you manage to keep it running.

It’s easy to understand, then, why fusion is often hailed as the harbinger of infinite, free energy for everybody — ever. So far, we’ve successfully recreated plasma in fusion reactors — the donut-shaped tokamaks or funky stellarators, for example — but we’ve yet to find a way of keeping this super-heated soup of charged particles stable for more than a few seconds.

One of the biggest hurdles we’ve encountered is that plasma in fusion reactors tends to fluctuate wildly in terms of temperature and density. Such turbulence is very dangerous, as any inkling of runaway plasma will eat through a reactor’s wall like a lightsaber through butter. Faced with such odds, researchers have little choice but to shut down experimental reactions before they run amok.

Plasma MAST Tokamak.

Plasma confined in the MAST tokamak at the Culham Centre for Fusion Energy in the UK. Magnetic field lines that combine to act like an invisible bottle for the plasma.
Image credits ITER / CCFE.

The most frustrating thing is that we know what we have to do, but not how to do it. We need to contain the plasma in an orderly fashion and keep the reaction going long enough for it to start being net-energy-positive — i.e. generate more energy than we put in.

Stars can cash in on their sheer mass to press plasma into playing nice, but we don’t have that luxury. Instead, we use massively-powerful magnetic fields (some 20,000 times stronger than that of the Earth) to keep it away from the reactor’s walls.

Go with the flow

This is where the present paper comes in. Certain types of plasma flows (like those inside stars) have been found to be very stable over time, without dangerous turbulence. We didn’t know how to make plasma flow like this, but the PPPL researchers report it comes down to a mechanism called magnetic flux pumping forcing the flow at the core of the plasma body to stay stable.

According to the flow simulations the team ran, magnetic flux pumping can take place in hybrid scenarios — a mix of the standard flow regimes currently known from theoretical and experimental models. These standard regimes include high-confinement mode (H-mode) and low-confinement mode (L-mode).

In L-mode, an electrically-balanced scenario — meaning it has a perfect ratio of positive to negative charged-particles — formed at lower temperatures, turbulence allows the plasma to leak away some of its energy. L-mode is unstable as high-temperature plasma at the core is thrown out to the surface, destabilizing the reaction. If this mode can be surpassed and the reaction enters H-mode, the overall temperature of the plasma body is increased and the reaction stabilizes. H-mode is an energy-imbalanced mode, but the plasma is kept stable and confined by electrical fields it itself generates (T. Kobayashi et al., Nature, 2016).

In a hybrid scenario, however, the flow is kept orderly only at the plasma body’s core. This generates an effect similar to that encountered inside the Earth, the team reports, where the solid iron core acts as a ‘mixer’, generating a magnetic field. The interactions between this field, the one applied by the generator, and the two types of plasma flow stabilize the reaction.

Even better, this magnetic flux pumping mechanism is self-regulating, the simulations show. If the mixer becomes too strong, the plasma’s current drops just below the point where it would go haywire.

And, even better #2, the authors suggest that ITER — widely held to be the most ambitious nuclear fusion project, currently under construction in Provence, France — may be suited to experiment with developing magnetic flux pumping by using the same hardware it employs to heat up the plasma.

The paper “Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations” has been published in the journal Physics of Plasmas.

NASA twin probes crash into the Moon

I have to say I’m a little sad writing this – engineers working at NASA‘s Jet Propulsion Laboratory (JPL) have received confirmation that the twin probes used to create the most accurate and detailed gravity map of the Moon have successfully crashed into the surface of our planet’s satellite. I say successfully because this is what they planned.

Artist’s depiction of Ebb and Flow

Ebb and Flow have helped NASA gain a broader understanding not only of how the Moon formed, but also how the entire solar system was in its early days, as part of the Gravity Recovery and Interior Laboratory (GRAIL) mission. They began their final rocket bursts an hit the Moon at around 2:28 p.m. PST (5:28 EST).

The probes crashed around the North Pole, bringing their successful mission to an end; sadly, because of their low orbit and low fuel reserves, it was the only thing that could be done with them. Still, even in their final hour, they provided valuable information: this time, about how much fuel they have left, enabling engineers to better calculate fuel consumption in future Moon missions.

GRAIL’s final resting place on the moon will be in shadow at the time of impact, so no video documentation of the impacts is expected.

Via NASA

New space pics show ‘battered’ Moon

The Ebb and Flow satellites, known together as the Grail mission are mapping the slight differences in gravity across the Moon. The results show that the Moon received a much bigger battering than previous expected.

A troubled past

Beneath its surface, the Moon’s interior bears clues from the very early solar system – unlike the Earth. While our planet is active from a tectonic point a view and constantly erases and recirculates almost any trace of the planet’s earlier composition, the Moon’s interior remained rather undisturbed as the billions of years passed, practically preserving a record of the rocks and processes that took place in its early history.

Now, researchers from MIT, NASA, and the Jet Propulsion Laboratory have found evidence than even beneath its surface, the Moon’s crust is almost completely pulverized. Their findings suggest that in its first billion years, the Moon (and probably the Earth too) suffered much more damage from celestial impacts than previously thought.

No escaping gravity

Using the data from GRAIL’s measurements, geophysicists have created a high resolution map of the moon’s gravity — a force created by surface structures such as mountains and craters as well as other geologic structures located beneath the surface.

Basically, they analyzed the gravity anomaly. You calculate the Moon’s gravitational attraction in difference points, and basically variations in gravity anomalies are related to anomalous density distributions within – thus enabling researchers to understand the internal structure of the planet (or satellite, depending on the case).

Lifting the veil

“It was known that planets were battered by impacts, but nobody had envisioned that the [moon’s] crust was so beaten up,” says MIT’s Maria Zuber, who leads the GRAIL mission and is the E.A. Griswold Professor of Geophysics in the Department of Earth, Atmospheric and Planetary Sciences. “This is a really big surprise, and is going to cause a lot of people to think about what this means for planetary evolution.”

Zuber and her colleagues detailed the findings in three different papers published in Science.

Aside from indicating a rough past, the map also highlighted numerous structures on the moon’s surface that were unresolved by previous gravity maps of any planet, including volcanic landforms, impact basin rings, and many simple, bowl-shaped craters. From the measurements, they indicated calculate a crust thickness of 34 to 43 kilometers, much thinner than previously estimated by geologists. Also, crust beneath some major basins is nearly nonexistent, indicating that impacts actually excavated the lunar mantle, providing a window into the interior – a window still easy to observe today.

In order to generate the gravity map, GRAIL’s two probes used a rather unorthodox technique – measuring changing distance between themselves as they orbit in tight formation around the moon. As a probe flies over a denser area, with a stronger gravitational pull, the stronger local gravity will pull that probe ahead, widening the space between the two spacecraft. As it flies over a less denser area, the reverse process happens; researchers used the changing distances between the two probes to translate it into the map.

The thing is that using this method, they map the effect of both the surface structures and the interior; in order to check only the moon’s interior, Zuber’s team used topographic measurements from another of their instruments, a laser altimeter aboard the Lunar Reconnaissance Orbiter, a separate spacecraft in orbit around the moon. They calculated the expected gravitational field judging by the topography, and then subtracted it from the measurements done by GRAIL.

“It’s essentially like removing a veil to reveal the gravity due to the inside of the planet,” Zuber says. “And when we saw those maps, we were just speechless.”

The interior map did reveal linear structures of denser material, which Zuber and her team believe to be buried lunar dikes – a type of magmatic sheet intrusion that seeped into fractures then solidified as rock. The dikes are an important piece of evidence, indicating the expansion of the moon in its earliest history. However, all in all, 98 percent of all the lunar crust is fragmented — a clear remnant of very early, very massive impacts.

“This is interesting for the moon,” Zuber says. “But what it also means is that every other planet was being bombarded like this.” The resulting fractures, she says, affect the way a planetary body loses heat and also provide a pathway for the transport of interior fluids.

Great data

Never has a study of this resolution been performed on the Moon.

“The staggering quality of the data reported by Professor Zuber and her colleagues is amazing,” says Kring, who was not involved in the research. “The data are exciting because they foretell far more insights than are captured in these initial three papers.”

But aside from the study itself, it has to be said that the biggest accomplishment is the ships themselves: Ebb and Flow have already provided incredibly valuable information – and they’ve just started.

“On this mission, with two spacecraft, everything had to go perfectly twice,” Zuber says, adding proudly: “Imagine you’re a parent raising a twins, and your children sit down at the piano and play a duet perfectly. That’s how it feels.”

Via MIT News